1. Field of the Invention
This invention relates to microcell electrochemical devices and assemblies, methods of making same by various techniques, and use of such devices and assemblies.
2. Description of the Art
In the field of energy supplies and energy conversion devices, and particularly in the development of fuel cells and batteries, there has been continuing effort to develop devices with significant power outputs (high current and/or high voltage), high power density, and high energy output per unit volume.
Structurally, electrochemical cells such as batteries and fuel cells are relatively simple, utilizing respective positive and negative electrodes separated in such manner as to avoid internal short circuiting, and with the electrodes being arranged in contact with an electrolyte medium. By chemical reaction at the electrodes, the chemical energy of the reaction is converted into electrical energy with the flow of electrons providing power when the electrode circuit is coupled with an external load.
Battery cells may use separator plates between respective electrodes so that multiple sheet elements are arranged in successive face-to-face assemblies, and/or such sheets may be wound together in a (spiral) roll configuration.
The fuel cell is of significant current interest as a source of power for electrically powered vehicles, as well in distributed power generation applications.
In fuel cells, a fuel is introduced to contact with an electrode (anode) and oxidant is contacted with the other electrode (cathode) to establish a flow of positive and negative ions and generate a flow of electrons when an external load is coupled to the cell. The current output is controlled by a number of factors, including the catalyst (e.g., platinum in the case of hydrogen fuel cells) that is impregnated in the electrodes, as well as the kinetics of the particular fuel/oxidant electrochemical reaction.
Currently, single cell voltages for most fuel cells are in the range of about 0.6-0.8 volts. The operating voltage depends on the current; as current density increases, the voltage and cell efficiency correspondingly decline. At higher current densities, significant potential energy is converted to heat, thereby reducing the electrical energy of the cell.
Fuel cells also may be integrated with reformers, to provide an arrangement in which the reformer generates fuel such as hydrogen from natural gas, methanol or other feed stocks. The resulting fuel product from the reformer then is used in the fuel cell to generate electrical energy.
Numerous types of fuel cells have been described in the art. These include:
polymer electrolyte fuel cells, in which the electrolyte is a fluorinated sulfonic acid polymer or similar polymeric material;
alkaline fuel cells, using an electrolyte such as potassium hydroxide, in which the KOH electrolyte is retained in a matrix between electrodes including catalysts such as nickel, silver, metal oxide, spinel or noble metal;
phosphoric acid fuel cells using concentrated phosphoric acid as the electrolyte in high temperature operation;
molten salt fuel cells employing an electrolyte of alkali carbonates or sodium/potassium, in a ceramic matrix of lithium aluminate, operating at temperatures on the order of 600-700 degrees C., with the alkali electrolyte forming a high conductive molten salt;
solid oxide fuel cells utilizing metal oxides such as yttria-stabilized zirconia as the electrolyte and operating at high temperature to facilitate ionic conduction of oxygen between a cobalt-zirconia or nickel-zirconia anode, and a strontium-doped lanthanum manganate cathode.
Fuel cells exhibit relatively high efficiency and produce only low levels of gaseous/solid emissions. As a result of these characteristics, there is great current interest in them as energy conversion devices. Conventional fuel cell plants have efficiencies typically in the range of 40-55 percent based on the lower heating value (LHV) of the fuel that is used.
In addition to low environmental emissions, fuel cells operate at constant temperature, and heat from the electrochemical reaction is available for cogeneration applications, to increase overall efficiency. The efficiency of a fuel cell is substantially size-independent, and fuel cell designs thus are scalable over a wide range of electrical outputs, ranging from watts to megawatts.
A recent innovation in the electrochemical energy field is the development of microcellsxe2x80x94small-sized electrochemical cells for battery, fuel cell and other electrochemical device applications. The microcell technology is described in U.S. Pat. Nos. 5,916,514; 5,928,808; 5,989,300; and 6,004,691, all to Ray R. Eshraghi. The microcell structure described in these patents comprises hollow fiber structures with which electrochemical cell components are associated.
The aforementioned Eshraghi patents describe an electrochemical cell structure in which the single cell is formed of a fiber containing an electrode or active material thereof, a porous membrane separator, electrolyte and a second electrode or active material thereof. Cell designs are described in the Eshraghi patents in which adjacent single fibers are utilized, one containing an electrode or active material thereof, the separator and electrolyte, with the second fiber comprising a second electrode, whereby the adjacent fibers constitute positive and negative electrodes of a cell.
The present invention embodies additional advances in the Eshraghi microcell technology.
This invention relates to microcell electrochemical devices and assemblies, methods of making same by various techniques, and use of such devices and assemblies.
The invention relates in one aspect to an electrochemical device comprising water-permeable membrane hollow fibers distributed in an assembly including a plurality of microcells potted at respective ends of the assembly and disposed within a housing wherein the potted respective ends bound an interior volume therebetween, and wherein the hollow fibers are parallely aligned with microcells of the assembly, with each hollow fiber having a first open end extending through the tubesheet exteriorly of the interior volume and the other end terminating at or before the opposite potting member, whereby the hollow fibers are arranged to absorb water produced in the electrochemical reaction by wicking action and channeling water away from the locus of electrochemical reaction by permeation through the wall of the hollow fiber and flow thereof through the bore of the hollow fiber to a collection locus in the housing outside of the interior volume. Another aspect of the invention relates to an electrochemical cell module, comprising:
a multiplicity of microcells in an assembly comprising a multiplicity of component microcell sub-bundles,
each microcell comprising an inner electrode,
a microporous membrane separator in contact with the inner electrode,
an electrolyte in pores of the microporous membrane separator,
an outer electrode,
with the microcell assembly including a plurality of hollow fiber heat exchange elements arranged for flow of an aqueous coolant medium through a central lumen thereof, with the hollow fiber heat exchange elements being distributed in the assembly for heat removal from the assembly during electrochemical reaction in operation of the module;
a source of the aqueous coolant medium;
flow circuitry interconnecting the source of said aqueous coolant medium and said hollow fiber heat exchange elements;
wherein the hollow fiber heat exchange elements comprise a water-permeable porous membrane separator, whereby water deriving from the aqueous coolant medium permeates from the bore through the membrane separator wall into the feed stream, thereby humidifying the electrochemical reaction environment.
A still further aspect of the invention relates to a method of water management in an electrochemical device including a plurality of microcells potted at respective ends and disposed within a housing wherein the potted respective ends bound an interior volume therebetween, such method comprising arranging hollow fibers to absorb water produced in the electrochemical reaction by wicking action, channeling water away from the locus of electrochemical reaction by permeation through the wall of the hollow fiber and flowing same through the bore of the hollow fiber to a collection locus in the housing outside of the interior volume.
Another aspect of the invention relates to a method of making an electrochemical cell module, comprising:
a multiplicity of microcells in an assembly comprising a multiplicity of component microcell sub-bundles,
each microcell comprising an inner electrode,
a microporous membrane separator in contact with the inner electrode,
an electrolyte in pores of the microporous membrane separator,
an outer electrode,
comprising fabricating the microcell assembly with a plurality of hollow fiber heat exchange elements arranged for flow of an aqueous coolant medium through a central lumen thereof, with the hollow fiber heat exchange elements being distributed in the assembly for heat removal from the assembly during electrochemical reaction in operation of the module;
wherein the hollow fiber heat exchange elements comprise a water-permeable porous membrane separator, whereby water deriving from the aqueous coolant medium permeates from the bore through the membrane separator wall into the feed stream, thereby humidifying the electrochemical reaction environment.
Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.